High performance radio navigation stationcus

ABSTRACT

A radio-navigation station allowing, in particular, the introduction of a wide-base system in a pre-existing station with a single antenna radiating a rotating cardioid radiation pattern, without replacement of the transmitting and receiving equipment. A plurality of such antennas are used each one being successively connected, for a predetermined duration, to the receiver in the case of a direction-finding station and to the transmitter in the case of a radio-beacon.

0 ilnited States Patent 1 [111 3,747,101 Becavin July 17, 1973 1 111011PERFoRMANCE 2,465,384 3/1949 Marchand 343/118 x 2,994,081 7/1961 Jordanet al.... 343/123 X RADIO NAVHGATION STATIONQUS 3,047,864 7/1962 Byatt33/118 X [75] Inventor: Henri G. Becnvin, Paris, France [73] Assignee:Thomson-CSF, Paris, France Primary Examiner Benjamin Ah Borcheh [22]Filed: June 30, 1971 Assistant Examiner-Richard E. Berger pp No: 158,486Attorney-Cushman, Darby & Cushman [30] Foreign Application Priority Data[57] ABSTRACT July 17, 1970 France 7026432 May 18, 1971 France 7117978 Aradio-navigation station allowing, in particular, the July 17, 1970France 7026433 introduction of a wide-base system in a pre-existing sta-May 18, 1971 France 71 17979 tion with a single antenna radiating arotating cardioid radiation pattern, without replacement of thetransmit- [52] [1.8. Ci. 343/106 R, 343/118 ting and receivingequipment. [51] Int. Cl. G018 1/54 A plurality of Such antennas are usedeach one-being [58] Field of Search 343/118, 106 R successivelyconnected, for a predetermined duration to the receiver in the case of adirection-finding station [56] References Cited and to the transmitterin the case of a radio-beacon.

UNITED STATES PATENTS 3,144,649 8/1964 Steiner 343/118 X '7 (Iiaims, 4Drawing Figures O O g\ *5 Race/yin 3%;???" 42 f 45 a 7 Clark L 4 1oPATENTEUJUL 1 7191s SHEEI 1 OF 4 Receiying dev/ce Clock geueratorConlrol signal Pmmwwm' 3,747,101.

mm 0F 4 Switching device I07 I k I Genera tons,

The present invention relates to radio-navigation, i.e.,direction-finding or beacon stations, and more particularly to wide-basestations that is to say those in which the receiving or transmittingpoints are distributed over a curve or a surface the dimensions of whichare at least comparable with the operation wavelength.

One of the essential causes of inaccuracy in radionavigation informationstems from the station environment which can disturb the propagation ofelectromagnetic waves through creating supplementary paths.

Among the known wide-base stations, some require an important number offixed antennas to produce a high-directivity lobe whose displacement isobtained either through using several groups of antennas which are fedin turn, or through varying the feeding of a single antenna group. Otherones (Doppler effect antennas) use a single antenna the radiationdiagram of which is fixed relatively to the antenna, but the antennabeing moved by mechanical means.

All these devices yield an appreciable reduction in the errors.

' However, where the problem is 'to improve an existing small-basestation, the introduction therein of a wide base requires a replacementof all the transmitting and receiving equipment.

The present invention overcomes this drawback, in the very common caseof a pre-existing station using a single antenna with a rotatinghorizontal cardioid radiation pattern, through making it possible tointroduce therein a wide base without any change in the transmitters andreceivers.

According to the invention, there is provided a radionavigation stationcomprising a plurality of antennas with a rotating cardioid radiationpattern, an amplitude modulation transmitting or receiving device, andswitching means for successively connecting said device to each of saidantennas for a predetermined duration.

The invention will be better understood and other of its featuresrendered apparent, with the help of the description and the attacheddrawings in which FIG. I schematically shows an embodiment of a radioD.F. station in accordance with the invention;

FIGS. 2 and3 are explanatory diagrams;

FIG. 4 schematically shows an embodiment of a radio-beacon station inaccordance with the invention.

Before describing the station shown in FIG. l, the operation of adirection-finding station using a single antenna with a cardioid cardoidradiation pattern will be briefly recalled.

The high frequency wave received from an aircraft to be localized isamplitude modulated at the rotation frequency F of the pattern, whichfrequency is generally comprised between 25 and 100 c/s the modulationlow frequency signal is detected in an AM receiver; it is thereaftercompared in phase with a reference signal which is synchronous with therotation of the pattern. The phase of the modulation signal relativelyto the reference signal, which hereinafter will be more briefly referredto as the phase of the low frequency signal, indicates the azimuth ofthe aircraft, which information is given to the observer by an indicatorsystem.

In order to obtain a rotating cardioid diagram, it is possible, inparticular, to use an antenna comprising a centre dipole feeding thereceiver, which centre dipole is surrounded by N reflector dipoleslocated at the apices ofa regular polygon with N sides, and one of whichonly is used at any one time, the others being then detuned. Throughusing all the reflectors successively, there is obtained an N positionrotating radiation pattern and the corresponding detected signal isfiltered to be converted into a sinusoidal waveform, the phase of whichis compared with that of the reference signal.

The direction-finding station shown in FIG. 1 comprises n six antennas,1 to 6 of the above described type. Each of those antennas thuscomprises a centre dipole, such as the dipole 11 of the antenna 1 and N6 reflector dipoles, such as the dipole 12 of the antenna 1 N being inthis example equal to n but this equality not being in the leastnecessary.

The antennas 1 4 and 6 are located at the apices of an equilateraltriangle inscribed .in a circle of centre 0. and the radius 2A, where Ais the operation wavelength.

The antennas 2 3 and 5 are each located at the centre of respectivesides of the triangles.

The antennas 2, 3 and 6 are thus located on a circle concentris with theone just described but of radius X.

Moreover, the height of the antennas above the ground is advantageouslychosen in such fashion as to cancel the field reflected by the ground,in the direction of the other antennas.

A receiving device 8 identical to that which is used in the stationswith a single antenna, comprises the receiver proper, the phasecomparator and the indicator system. A clock It) delivers at its output41 pulses I the recurrence frequency of which isf= 1/9, where 9 is theduration for which a dipole reflector is placed in the resonancecondition, and, by means of a frequency divider incorporated therein,delivers at its outputs 42 and 43 pulses J the recurrence frequency ofwhich is the rotation frequency F of the cardioid radiation patterns. Agenerator 7 provides the control signals for placing the reflectordipoles in the resonance state and receives to this end the pulses I Aswitching device 9 receives the pulses J from the output 42 of the clockill).

The pulses J delivered at the output 43 of the clock are supplied to thereceiving device 8 for the generation of the phase reference signal.

The diode of each one of the reflectors is connected In a firstembodiment, each antenna is successively made operative for a duration Tequal to the period of rotation of the radiation pattern, while thereflectors thereof are successively put in the resonance state for aduration 9 T/N. In that case f= NF.

By means of the synchronizing pulses l the generator 7 delivers at Noutputs, grouped in a single cable in the Figure, N series of pulses Sthe series S,(i 1,2 N) being affected to the dipoles whose positionnumber is i in each of the antennas. The pulses of each series have aduration T/N and a period T the pulses of the series S, being shifted byG relatively to the pulses of the series S The'switching device 9 bymeans of the pulses J the frequency of which is F effects in the courseof a cycle of duration n T the following operations In the course of thej'(j=l,2 n) time interval of duration T the switching device 9 directsthe N series S, respectively to the corresponding reflectors of the j"antenna at the same time as it places the center dipole of this antennain the resonance state and connects it to the receiving device 8 thesynchronization of the rotation of the radiation pattern with the phasereference signal being ensured by means of the pulses J The lowfrequency signal forming the envelope of the signal received by thereceiver presents in the course of a complete cycle of duration n Tsuccessively the following phases (relatively to the phase referencesignal) P AP P AP P AP, where AP, is the error affecting the j" antenna;this signal is averaged by the low frequency filtering of the receiverand by the indicator system following the phase comparator, theindicator system thus delivering an information corresponding to thephase P AP, where AP AP,+AP AP,,)/n which on an average considerablydecreases the final phase error.

The averaged errors are essentially those resulting from the creation ofreflected rays from the environment, these rays arriving at the antennaat a different angle to the principal ray.

Calculation shows that under certain conditions this mean error issubstantially less than that encountered with a single antenna, theratio K between the two errors being as small as U10 and even less.

In FIG. 2 the value of this factor K as a function of the angle abetween the direction of the reflected parasitic ray and the principalray, when an antenna is assumed to rapidly and successively occupy allthe points on a surface or a curve, has been illustrated, under theassumption that a as well as the amplitude A of the reflected signal,remains the same for each of these points.

A curve has been plotted making the assumption that the antennas aredistributed upon the surface of a circle of radius equal to twice thewavelength of the high-frequency signal being received. The curve 21assumes distribution on a circumference of the same radius and the curve22 on two circumferences of respective radii equal to said wavelengthand to twice said wavelength.

The improvement thus obtained is a really major one, in the order of 8times (K 0.125) on average, but this is a theoretical result since itassumes the use of an infinite number of antennas.

In FIG. 3 the value of this same factor is illustrated as a function ofthe angle a for the case where the number of antennas is finite. Thecurve 32 relates to an array of six antennas in accordance with FIG. 1and the curve 31 corresponds to the case where the radius of theexternal circle passing through three antennas 1, 4, 6 is equal to only1.6 times the wavelength.

It will be seen that a gain of 3 (K 0.33) can on an average be obtainedwith this design.

In practice, the reflected rays come from specific directions due to thepresence of large-sized obstacles such as hangers and hills, and it istherefore possible by appropriate orientation of the antenna system toobtain much better real improvement factors, better than five forexample.

The same applies to the kinds of antennas used, which may be of anyknown type producing a radiation pattern in the form of a rotatingcardioid.

The described device assumes however that the inertia of the indicatorsystem does not allow it to follow fluctuations of frequency PM l/nTwhich may lead to limit n or to eliminate particular indicator systems.

It is possible to avoid such limitations through effecting the switchingoperations so that all the antennas are successively made operative inthe course of a time interval n9 shorter than nT and increasing thus thefrequency f of the fluctuations to a value 1 /n9 sufficiently high forensuring the entire elimination of the fluctuations by the filtering inthe low frequency stages of the receiver.

With the antenna system of FIG. 1 the switching operation may forexample be effected in the following way.

A whole cycle has a duration T divided in N time intervals of durationT/N in the course of which one and the same position of the rotatingradiation pattern is used successively for each of the antennas.

The fundamental frequency of the fluctuations is then NF (instead ofFlu) and the filtering of the low frequency stages of the receiver willsuffice to eliminate those fluctuations, whatever the indicator systemmay be.

In that case, the clock 10 delivers at its output 41 pulses with thefrequency nNF and at its output 42 pulses with the frequency NF Thegenerator 7 delivers at n separate outputs n series U,( 1,2 n) of pulsesof duration 9 T/n with the period T/N the pulses of the series U, beingshifted by G relatively to those of the series U The pulses of the nseries U, are respectively used by switching device 9 to place in theresonance state the center dipoles of the n antennas respectively, toconnect those dipoles to the receiver, and to place in the resonancestate one of the reflectors.

The reflector used for each antenna is successively the reflector withposition number 1 (in the first time interval of duration T/N of eachcycle), then the reflector with position number 2 and so on, thenecessary switching being effected in the switching device 9 by means ofthe pulses of frequency NF.

FIG. 4 shows a radio-beacon station according to the invention.

The operation mode of a radio-beacon using a single antenna with arotating cardioid diagram will first be briefly recalled. Such a diagrammay be obtained for example through using a complex antenna, comprisingtwo crossed dipoles and an omnidirectional antenna.

The latter is fed by a first HF signal amplitude modulated by asubcarrier which is itself frequency modulated by the phase referencesignal at the frequency F the frequency of which is generally of theorder of 30 Hz; the two dipoles respectively receive two other HFsignals, having the same angular frequency a) and the same HF phase asthe first one, and having respectively the form sin 21rFt sin wt and cos21rFt sin wt.

The first signal is radiated according to an omnidirectional pattern,and the subcarrier thereof supplies the phase reference; the combinationof the two other radiated HF signals gives a rotating radiation patternat the frequency F which combined with the omnidirectional pattern givesthe cardioid pattern.

Reverting to FIG. 4 six antennas Hill to 106 are respectively located inrelation to one another like the six antennas l to 6 of FIG. ll each ofthose antennas is schematically represented by two crossed dipoles.

Conventional signal generators 108, 109, 110 respectively deliver the HFsignal for the omnidirectional antennas (not shown in the figure) andthe two signals which are modulated in phase quadrature. A switchingdevice 107 fed by the three signal generators is connected to theantennas 1100 to 106 by means of cables 171 to 176 respectively.

Each of these cables comprise three transmission lines respectivelydelivering the three HF signals to the antennas.

This system may be operated in the two ways indicated for thedirection-finding station.

In a first embodiment, the n antennas are successively fed, each for aduration T l/F through the switching device 107 which receives, to thisend, by means of a connection not shown in the Figure, a signal at thefrequency F of the modulating signal, a complete cycle of operationhaving the duration nT.

In a second embodiment, the n antennas are successively made operativefor a duration 9 T/p smaller than T The number p must be so chosen thatthe fundamental and harmonic frequencies generated by the switching ofthe antennas should lie not only outside the low frequency band used forthe determination of the azimuth, but also outside the low frequencybands used (by means of auxiliary modulation of the HF signals) foradditional information (identification of the beacon, telephony). On theother hand, a choice of an integer for the value ofp simplifies theswitching. The device 107 receives then a signal at the frequency pFfrom which the frequency F of the modulating signal is advantageouslyderived by frequency division.

Relatively to the usual operation with a single antenna the aircraftreceivers are not in the least affected by the new structure of thebeacon.

' As concerns the averaging of the errors, the considerations made inthe case of a direction-finding station also apply in this case.

The triangular arrangement shown in FIGS. l and 4 and the spacingsindicated are of course only one of the numerous arrangements which arepossible; a diamond-shaped arrangement in particular brings a gain ofbetter than ten if the parasitic ray is located in a narrow sectormaking an angle of 1 10 with the main effective signal. The choice ofthe arrangement is a function of the special features of the site wherethe antenna is to be installed. In the case ofa well defined andlocalized obstacle, it may be advantageous to use a linear arrangementof the antennas, for example, normally to the direction of the obstacle.

The same applies to the kinds of antennas used, which may be of anyknown type producing a radiation pattern in the form of a rotatingcardioid.

It is very clear from the foregoing that the radionavigation station inaccordance with the invention makes it possible to obtain performancecharacteristics which are at least as good as those of Doppler stations,and it has the advantage over these latter of much greater simplicity,much lower cost and, above all, of being able to be developed readilyfrom existing stations having only one antenna.

What is claimed is ll. A radio-navigation station comprising a pluralityof antennas each with a rotating cardioid radiation pattern, theradiation patterns of said antennas having a common rotation period T anamplitude modulation device, and switching means for successivelyconnecting said device to each of said antennas for a predeterminedduration.

2. A radio-navigation station as claimed in claim 1, wherein saidpredetermined duration is equal to the rotation period T of saidradiation patterns.

3. A radio-navigation station as claimed in claim 1, wherein saidpredetermined duration is shorter than the rotation period T of saidradiation patterns.

4. A radio-navigation station as claimed in claim 1 for transmittingdirection-indicating signals, said amplitude-modulation device being atransmitting device.

5. A radio-navigation station as claimed in claim 1 for receivingdirection-indicating signals, said amplitude-modulation device being areceiving device.

6. A radio-navigation station as claimed in claim 5, wherein saidrotating radiation patterns being patterns with N discrete positions,said predetermined duration is equal to T/(n.l\l)n being the number ofantennas. I

7. A radio-navigation station as claimed in claim 5, wherein each ofsaid antennas comprises a vertical centre dipole and several peripheralvertical dipoles located at the apices of a regular polygon, each ofsaid dipoles being interrupted by a diode, the conduction of whichplaces said dipoles in the resonance state relatively to the operationwavelength, said switching means ensuring in addition the control ofsaid diodes so that only the center dipole and one of the peripheraldipoles of the antenna being connected to said device is placed in theresonance state.

1. A radio-navigation station comprising a plurality of antennas eachwith a rotating cardioid radiation pattern, the radiation patterns ofsaid antennas having a common rotation period T , an amplitudemodulation device, and switching means for successively connecting saiddevice to each of said antennas for a predetermined duration.
 2. Aradio-navigation station as claimed in claim 1, wherein saidpredetermined duration is equal to the rotation period T of saidradiation patterns.
 3. A radio-navigation station as claimed in claim 1,wherein said predetermined duration is shorter than the rotation periodT of said radiation patterns.
 4. A radio-navigation station as claimedin claim 1 , for transmitting direction-indicating signals, saidamplitude-modulation device being a transmitting device.
 5. Aradio-navigation station as claimed in claim 1 , for receivingdirection-indicating signals, said amplitude-modulation device being areceiving device.
 6. A radio-navigation station as claimed in claim 5,wherein said rotating radiation patterns being patterns with N discretepositions, said predetermined duration is equal to T/(n.N)n being thenumber of antennas.
 7. A radio-navigation station as claimed in claim 5,wherein each of said antennas comprises a vertical centre dipole andseveral peripheral vertical dipoles located at the apices of a regularpolygon, each of said dipoles being interrupted by a diode, theconduction of which places said dipoles in the resonance staterelatively to the operation wavelength, said switching means ensuring inaddition the control of said diodes so that only the center dipole andone of the peripheral dipoles of the antenna being connected to saiddevice is placed in the resonance state.